1. Field of the Invention
The present invention relates to a semiconductor memory device equipped with a semiconductor memory unit and, more particularly, to a semiconductor memory device equipped with a transfer mode control unit which changes a transfer mode for data transfer to/from a host device.
2. Description of the Related Art
Recently, semiconductor memory device such as flash memory cards which are non-volatile semiconductor storage media have been developed and put into widespread use as external storage devices for information devices such as digital cameras which are host devices. With increases in amounts of data handled by the host devices, flash memories have been growing in packaging density and storage capacity.
NAND-type flash memory, which features large storage capacity, is a type of flash memory which has recently been used particularly widely.
With the NAND-type flash memory, charge injected from a channel to a charge storage layer via an insulating film by means of a tunnel current is used as a digital bit information storage unit and information is read out by measuring conductance changes of a field-effect transistor based on the amount of charge. Unlike a DRAM, data can be read from the NAND-type flash memory multiple times without data corruption.
High read and write speed is required of semiconductor memory device, requiring a high bus transfer rate of a transfer bus. Therefore, for example, high speed specification is prescribed for a memory card bus with its transfer clock frequency raised from 25 MHz of normal mode to 50 MHz of high speed mode, enabling a higher data transfer rate.
Furthermore, for a higher data transfer rate, Japanese Patent Application Laid-Open Publication No. 2007-11788 discloses a memory card which provides an ultra-high speed mode capable of obtaining twice the data transfer rate using the same clock frequency as a high speed mode by transmitting and receiving data in sync with rising edges and falling edges of a clock signal supplied from a host device.
Memory cards have been designed to support various transfer modes, prescribed in advance, for data transfer with host devices. For example, the SD Memory Card (registered trademark) standard (SD standard) provides an SD mode and SPI mode. The SD mode further includes a 1-bit mode and 4-bit mode to accommodate different data transfer bus widths. Furthermore, each of the above modes includes a normal mode, high speed mode, and ultra-high speed mode which differ in the transfer rate.
The memory cards which support various transfer modes are convenient for users. However, complex control strategies are required to perform control in all the transfer modes. Also, the memory cards may become larger and more complex. On the other hand, a memory card which does not support transfer modes prescribed in advance is not convenient for the user because when trying to start data transfer in a transfer mode not supported by the host device, the memory card may cause errors such as a freeze-up.
Incidentally, there is a method which increases the transfer clock frequency of the memory card to increase the data transfer rate. However, increases in the transfer clock frequency can result in the need to shield spurious radiation of electromagnetic waves, i.e., take measures for EMI (Electro Magnetic Susceptibility). Also, increases in the transfer clock frequency will cause increases in power consumption of the memory card.